Plaster mold for casting readily oxidizable metals



t r 2,802,747 Patented Aug. 13, 1957 PLASTER MOLD FOR CASTING READILY OXIDIZABLE METALS Application October 26, 1955, Serial No. 543,015

5 Claims. (Cl. 106-3855) No Drawing.

This invention relates to inhibiting any reaction between gypsum plaster molds or parts of molds and readily oxidizable molten metal coming in contact with the mold or mold part. The invention is more particularly concerned with the casting of magnesium and magnesium base alloys in gas permeable gypsum plaster molds.

Gypsum plaster, also known as plaster of Paris, has long been used to make special purpose molds for the production of metal castings. However, it was not until the introduction of plaster molds having a high permeability to the passage of gas, on the order of 30 to 70 American Foundrymens Society standard scale, that light metals could be satisfactorily cast in such molds. Even though the permeability was high enough, one drawback to employing plaster molds was encountered, namely, the occurrence of a reaction between the plaster and such readily oxidizable molten metal as magnesium and magnesium base alloys.

The tendency of molten magnesium and magnesium base alloys to react with water in green sand molds has been recognized for many years and this has been over come by incorporating in the sand mixture small amounts of certain substances such as sulphur, boric acid and decomposable fluorides. However, when added to gypsum plaster, these substances are not completely effective in preventing reaction, especially under severe conditions where, as in a mold riser, the molten metal remains in contact with the plaster for an extended period of time. The reaction appears to occur not only between the molten metal and water in the mold, but between the metal and the plaster, and hence is more difficult to overcome than in the case of green sand molds.

It is an object of this invention to substantially completely inhibit reaction between gypsum plaster and easily oxidizable metals, especially molten magnesium and magnesium base alloys. Another object is to provide a permeable gypsum plaster mold or mold part which is not attacked by readily oxidizable metals in the molten condition. A particular object is to provide a permeable gypsum plaster mold which will not react with molten magnesium or magnesium base alloys under severe conditions.

My invention is predicated upon the discovery that the presence of a small amount of a water insoluble substance selected from the group consisting of polychlorinated benzene, polyphenyl and condensed ring aromatic hydrocarbons in a hydrated gypsum plaster mold or mold part along with one or more conventional oxidation inhibitors substantially eliminates any reaction between the molten metal and the plaster, even under severe conditions. The mold or mold part may therefore be considered as being inert or highly stable toward the molten metal. For the purpose of convenience I refer to the polychlorinated benzene and poly-ring hydrocarbons as being supplemental inhibitors. It is significant that the supplemental inhibipreferred.

tors by themselves do not prevent the adverse reaction but in combination with other inhibitors the reaction is stopped or reduced to such a low level as to be insignificant. The quantity of supplementary inhibitors necessary to efiect substantial freedom from reaction will vary with the nature and amount of the other inhibitors employed but in general, from 1 to 5% of the weight of the dry plaster mixture prior to the addition of water is adequate.

The oxidation inhibitors to be used in association with the chlorinated organic compounds are one or more of the group consisting of sulphur, boric acid, alkali fluorides, alkali fluoborates and alkali silicofluorides. The amount of inhibitors to be used will vary with the thickness of the mold section and the mass of metal held in the mold. Obviously where molten metal remains in contact with the mold for a relatively long period of time it is necessary to provide more protection against reaction than where freezing occurs very quickly. Moreover, better results are generally obtained by using two or more inhibitors such as boric acid and alkali fluoborate or boric acid and alkaline silicofluoride or an alkali fluoborate and sulphur. Of the alkali fluoborates, potassium fiuoborate is In referring to alkali compounds, it is to be understood that this term includes the corresponding ammonium compounds. For most purposes, from 0.5 to 5% by weight of the dry plaster mixture of the inhibitor is sufiicient, and where two or more are employed, the total amount should not exceed about 8% of the weight of the dry mixture.

The supplemental inhibitor, as mentioned above, may consist of polychlorinated benzene, polyphenyl or condensed ring aromatic hydrocarbon. Examples of polychlorinated benzene are dichloro-, trichloro-, tetrachloro-, pentachloroand hexachlo-robenzene. The last named compound is preferred. The polychlorinated polyphenyl compounds are of the type represented by the dichloroand higher chlorine substituted products of diphenyl, triphenyl and higher phenyl groupings. Preferably they should be chlorinated to the extent of replacing at least three hydrogen atoms with chlorine. The polychlorinated condensed ring aromatic compounds are of the type illus trated by the dichloroand higher chlorine substituted products of naphthalenes, preferably those containing at least three chlorine atoms. Condensed ring compounds containing more than two rings, such as anthracene, may be employed, but they are more expensive. A single chlorinated compound, such as hexachlorobenzene, may be used to supplement the conventional inhibitor but it is often desirable to employ two or more of the chlorinated organic materials. For example, a mixture of 10% trichloro-, 50% tetrachloroand 40% pentachloro-naphthalene can be used. It is essential that the chlorinated compounds vaporize or decompose in contact with the molten metal, but they should remain in stable form in the plaster mold before the metal is introduced. Also, they should not vaporize during any mold drying operation. Their boiling points should not be, below 325 F. While on the other hand, they should readily vaporize or decompose below the melting point of the metal being cast. All of these compounds are insoluble in water, and hence precautions must be taken to thoroughly mix them with the dry plaster and water slurry before the mold is formed.

The plaster containing the inhibitors may constitute the entire mold or only part of a mold. Generally plaster molds are used where high accuracy and a very smooth surface are required on the metal castings.

Plaster mold parts are often employed where it is desirable to take advantage of the heat insulating property of the plaster, such as in gates and risers. The mold parts can be used in combination with both sand and permanent molds.

The base material usedin the production of the plaster molds is the familiar calcined gypsum,

CaSOsJ/zHzO which when fully hydrated yields the product CaSO4.2H2O

Although the kettle boiled type of calcined gypsum can be employed, I prefer to use that variety known in the trade as alpha gypsum, such as produced in accordance with teachings of U. S. Patent 1,901,051. Alpha gypsum offers the advantages of developing a greater strength and density and requiring less water to produce a normal pouring consistency than the kettle calcined material. For the purpose of diluting the calcined gypsum or developing particular properties in a mold or mold part, it is a Well known practice to admix the gypsum with filler materials such as talc, asbestos or silica. In general, not more than 75% by weight of the dry plaster mix should be composed of fillers and usually more than 1% is sufiicient. In addition to the fillers it is frequently desirable to include accelerators or retarders of setting of the calcined gypsum such as sodium sulphate, sodium chloride, sodium citrate and citric acid. These substances are commonly used in amounts of less than 1% by weight of the dry plaster mix.

The oxidation inhibitors and supplemental inhibitors are preferably added to the dry plaster and thoroughly mixed therewith prior to the addition of water as described below.

The desired permeability of the plaster molds may be developed in different ways. A water slurry of the plaster may be aerated and then allowed to set thereby creating a light weight mass filled with air bubbles. According to another method, the slurry may be poured without any aeration and the solidified product subsequently recrystallized and rehydrated as described in U. S. Patent 2,220,703. Where the slurry is to be aerated, a small quantity of foaming agent should be incorporated in the mix which permits the production and retention of air bubbles. Sulfonated hydrocarbons, as a class, are suitable for this purpose and sodium salts of alkyl aryl sulfonates are preferred. From about 0.1 to 1% by weight of these materials with respect to the weight of water employed is generally sufiicient to impart the desired foaming characteristics to the slurry.

To obtain a good pouring consistency in the slurry, the water or the solution containing any soluble components should be added to the dry plaster in the proportion of about 8 to 11 parts of the former to 8 to 11 parts of the latter. The proportions will vary somewhat according to the composition of the mold mixture, but generally the proportions necessary to provide a satisfactory pouring consistency will be obtained by observing the foregoing. Where the slurry is to be aerated before being poured, it is stirred with a suitable device which serves to introduce air and create small bubbles and uniformly distribute them throughout the slurry. If desired, a foam may be separately produced with water and a foaming agent and the foam added to the dry plaster mix. In any event, the volume of the slurry should be substantially increased by generation of the foamed condition. To obtain the desired permeability in the final product, it is advisable to increase the volume of the slurry by at least 25% and in many cases by as much as 150 to 200%. The permeability thus obtained may vary between about and 100 on the American Foundrymens Society scale of permeability.

As mentioned hereinabove, the calcined gypsum com- 2,so2,747 J bines with water to form the fully hydrated calcium sulphate to the mold. In addition to the water of hydration there may be a small amount of free or uncombined water present in the final product depending upon the thoroughness of the drying operation. It has been found that the presence of as much as 5% by weight of free water in the permeable mold does not destroy its utility and it may even be advantageous under some circumstances, but enough of the oxidation inhibitor must be present to inhibit any reaction between the water and the metal. Even if all the free water is driven off, as well as a portion of the combined water, the inhibitors must still be used to eliminate or minimize any reaction with the molten readily oxidizable metal.

In the production of permeable plaster molds, it is a common practice to dry the mold or mold part before it is used and thereby eliminate substantially all of the uncombined water. In so doing it is not uncommon for the water soluble inhibitors to migrate to the surface of the mold and form a crust thereon when the water is evaporated. Such a crust is undesirable in that it creates an uneven surface with a corresponding adverse effect upon metal castings poured in the mold. The adverse effect of migration and crust formation can be overcome by incorporation of from 0.2 to 5% by weight of urea formaldehyde in the dry plaster or in the slurry, as disclosed and claimed in copending application Serial No. 276,685, now U. S. Patent No. 2,753,608.

As mentioned hereinabove, the invention finds particular application in the production of magnesium and magnesium base alloy castings. Other metals and alloys which are easily oxidizable, such as aluminum base alloys containing substantial amounts of magnesium are reactive toward gypsum, may be satisfactorily cast in the same type of mold. In the case of magnesium and magnesium base alloys, it is helpful to add from 0.0001 to 0.05% by weight of beryllium to improve the resistance to oxidation of the molten metal.

The molten metal may be at any temperature within the range normally employed in pouring castings, namely, about 1300 to 1500 F.

The advantage gained from the use of the supplemental oxidation inhibitor is illustrated in the following examples where different inhibitors were employed. The test consisted of preparing cylindrical permeable hydrated plaster cuts 6 /2 inches high with an outside diameter of 3% inches and a wall thickness of inch. The cups were set to substantially their full height in a mass of loose shredded asbestos to reduce heat loss when the cups were filled with molten metal. The metal used in the tests consisted of the commercial magnesium base alloy nominally composed of magnesium, 6% aluminum, 3% zinc, and 0.2% manganese. The alloy was heated to 14001500 F. and poured into the cups, filling them almost completely. The metal was left in the cups for a period of 40 minutes, during which time it, of course, solidified. In this manner the metal remained molten for a longer time than occurs in normal casting practice, and hence any tendency for a reaction to occur could be detected. After the specified lapse of time, the cups were inverted to remove the solid metal slugs and the cups examined for evidence of any reaction. The plaster from which the cups were made was prepared by adding the inhibitors to the plaster in the proportions indicated below, adding 10 parts of water with foaming agent dissolved therein to 9 parts of dry plaster mix and aerating the slurry with an increase in volume of The molded cups were dried at 180 F. to eliminate any free water. The cups produced in this manner had a permeability of about 40 on the AFS scale. The amount and kind of inhibitors used and the evidence of reaction are set forth in the table below. The percentages of the inhibitors are based upon the proportion in the dry plaster mixture prior to the addition of water to form the plaster slurry.

Efiect of oxidation inhibitors on reactions with plaster cups From the foregoing it is evident that the chlorinated organic compound as the sole oxidation inhibitor was ineffective, but in combination with conventional inhibitors it almost completely prevented reaction with the plaster. Also, potassium fluoborate, with or without sulphur did not prevent reaction. The slight reaction observed in some cases was so small as to be insignificant from a commercial standpoint and the plaster is therefore considered to be substantially inert toward the molten metal.

Having thus described my invention and certain embodiments thereof, I claim:

1. A permeable gypsum plaster mold or mold part having as essential components thereof hydrated gypsum, from 0.5 to 5% by weight of the dry plaster mixture of at least one oxidation inhibitor selected from the group consisting of sulphur, boric acid, alkali fluorides, alkali fiuoroborates and alkali silicofluorides, and from 1 to 5% by weight of the dry plaster mixture of a supplemental oxidation inhibitor which is a polychlorinated hydrocarbon selected from the group consisting of henzene, polyphenyl and condensed ring aromatics, said supplemental inhibitors having a boiling point above 325 B, said mold or mold part being characterized by greater freedom from reaction when in contact with molten easily oxidizable metals than the same mold or mold part devoid of said supplemental oxidation inhibitors.

2. A permeable plaster mold or mold part having as essential components thereof hydrated gypsum, from 0.5

to 5% by Weight of the dry plaster mixture of at least one oxidation inhibitor selected from the group consisting of sulphur, boric acid, alkali fluorides, alkali fluoborates and alkali silicofiuorides and from 1 to 5% by weight of the dry plaster mixture of a supplemental oxidation inhibitor which is a polychlorinated hydrocarbon selected from the group consisting of benzene, polyphenyl and condensed ring aromatics, said polychlorinated hy- G drocarbons containing at least three chlorine atoms, said supplemental oxidation inhibitors having a boiling point above 325 F., said mold or mold part being characterized by greater freedom from reaction when in contact with molten easily oxidizable metals than the same mold or mold part devoid of said supplemental oxidation inhibitors.

3. A permeable gypsum plaster mold or mold part having as essential components thereof hydrated gypsum, from 0.5 to 5% by weight of the dry plaster mixture of boric acid, 0.5 to 5% by weight of the dry plaster mixture of potassium fluoborate, 0.5 to 5% by weight of the dry plaster mixture of sulphur, the total amount of said boric acid, potassium fluoborate and sulphur not exceeding 8%, and from 1 to 5% by Weight of the dry plaster mixture of hexachlorobenzene, said mold or mold part being characterized by greater freedom from reaction when in contact with molten easily oxidizable metals than the same mold or mold part devoid of said hexachlorobenzene.

4. A permeable gypsum plaster mold or mold part having as essential components thereof hydrated gypsum, from 0.5 to 5% by Weight of the dry plaster mixture of potassium fluoborate, 0.5 to 5% by weight of the dry plaster mixture of sulphur, the total amount of said potassium fluoborate and sulphur not exceeding 8%, and from 1 to 5% by weight of the dry plaster mixture of hexachlorobenzene, said mold or mold part being characterized by greater freedom from reaction when in contact with molten easily oxidizable metal than the same mold or mold part devoid of said hexachlorobenzene.

5. A permeable plaster mold or mold part having as essential components thereof hydrated gypsum, from 0.5 to 5% by Weight of the dry mixture of potassium fluoborate and from 1 to 5% by Weight of the dry plaster mixture of a supplemental oxidation inhibitor which is a polychlorinated hydrocarbon selected from the group consisting of benzene, polyphenyl and condensed ring aromatics, said supplemental inhibitors having a boiling point above 325 F., said mold or mold part being characterized by greater freedom from reaction when in contact with molten easily oxidizable metal than the same mold or mold part devoid of said supplemental oxidation inhibitors.

References Cited in the file of this patent UNITED STATES PATENTS 2,045,913 Hoy et al June 30, 1936 2,480,896 Bean Sept. 6, 1949 2,487,207 Adams Nov. 8, 1949 2,529,835 Dailey et al Nov. 14, 1950 FOREIGN PATENTS 479,406 Great Britain Feb. 4, 1931 

1. A PERMEABLE GYPSUM PLASTER MOLD OR MOLD PART HAVING AS ESSENTIAL COMPONENTS THEREOF HYDRATED GYLPSUM, FROM 0.5 TO 5% BY WEIGHT OF THE DRY PLASTER MIXTURE OF AT LEAST ONE OXIDATION INHIBITOR SELECTED FROM THE GROUP CONSISTING OF SULPHUR, BORIC ACID, ALKALI FLUORIDES, ALKALI FLUOROBORATES AND ALKALI SILICOFLUORIDES, AND FROM 1 TO 5% BY WEIGHT OF THE DRY PLASTER MIXTURE OF A SUPPLEMENTAL OXIDATION INHIBITOR WHICH IS A POLYCHLORINATED HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF BENZENE, POLYPHENYL AND CONDENSED RING AROMATICS, SAID SUPF., SAID MOLD OR MOLD PART BEING CHARACTERIZED BY GREATER FREEDOM FROM REACTION WHEN IN CONTACT WITH MOLTEN EASILY OXIDIZABLE METALS THAN THE SAME MOLD OR MOLD PART DEVOID OF SAID SUPPLEMENTAL OXIDATION INHIBITORS. 